#' Generate Global Tiling System
#'
#' @description
#' This function generates a matrix with bounding box information for a global
#' tiling system (based on Lat/Lon).
#'
#' @param tileSize `numeric`, size of a single tile in degrees (EPSG:4326).
#' @param offset `numeric`, shifts the tiling system in upper-left direction.
#' @param StartNameFrom `numeric`. `c(Lat-Direction,Lon-Direction)` start number
#' in the naming of the tiles.
#' @param extent `list`. Tile system extent information, basically the coverage
#' of the data on server.
#'
#' @return
#' A `matrix`.
#'
#' @author
#' Matteo Mattiuzzi
#'
#' @seealso
#' [getTile()].
#'
#' @examples
#' # 1x1 degree tiling system
#' e1 <- genTile()
#' head(e1)
#'
#' # 10x10 degree tiling system with offset to be aligned to Geoland2 Dataset
#' e2 <- genTile(tileSize = 10, offset = (1/112) / 2)
#' head(e2)
#'
#' # Tiling system for SRTMv4 data (CGIAR-CSI)
#' e3 <- genTile(tileSize = 5, StartNameFrom = c(1, 1),
#' extent = list(xmin = -180, xmax = 180, ymin = -60,ymax = 60))
#' head(e3)
#'
#' @export genTile
#' @name genTile
genTile <- function(tileSize = 1, offset = 0, StartNameFrom = c(0, 0),
extent = list(xmin = -180, xmax = 180, ymin = -90, ymax = 90))
{
# offset is used in case of pixel centrum reference. In such case the offset is res/2
if (offset!=0) {cat("Warning! Tiles crossing LAT extremas (-90 and +90) are not meaningful for now! For those tiles the resulting shift in LON is not computed!\n")}
# set origin in UL
LON <- seq(extent$xmin,extent$xmax,by=tileSize)
LON <- LON[-length(LON)]
LAT <- seq(extent$ymax,extent$ymin,by=-tileSize)
LAT <- LAT[-length(LAT)]
LON <- LON - offset
LAT <- LAT + offset
tiles <- expand.grid(LON,LAT)
colnames(tiles) <- c("xmin","ymax")
iv <- (0:(length(LON)-1)) + StartNameFrom[2]
ih <- (0:(length(LAT)-1)) + StartNameFrom[1]
vh <- expand.grid(iv,ih)
tiles$iv <- vh[,2]
tiles$ih <- vh[,1]
tiles$xmax <- tiles$xmin + tileSize
tiles$ymin <- tiles$ymax - tileSize
tiles[tiles$xmin < -180,"xmin"] <- tiles[tiles$xmin < -180,"xmin"] + 2*180
tiles[tiles$xmin > 180,"xmin"] <- tiles[tiles$xmin > 180,"xmin"] - 2*180
tiles[tiles$xmax < -180,"xmax"] <- tiles[tiles$xmax < -180,"xmax"] + 2*180
tiles[tiles$xmax > 180,"xmax"] <- tiles[tiles$xmax > 180,"xmax"] - 2*180
#TODO: EXTREMAS IN LAT are changing LON +- 180
# tiles[tiles[,"lat"] < -90,"lon"] <- tiles[tiles[,"lat"] < -90,"lon"] + 180 #
tiles[tiles$ymax < -90,"ymax"] <- -90 + abs(90 + tiles[tiles$ymax < -90,"ymax"])
tiles[tiles$ymax > 90,"ymax"] <- (2*90) - tiles[tiles$ymax > 90,"ymax"]
tiles[tiles$ymin < -90,"ymin"] <- -90 + abs(90 + tiles[tiles$ymin < -90,"ymin"])
tiles[tiles$ymin > 90,"ymin"] <- (2*90) - tiles[tiles$ymin > 90,"ymin"]
tiles <- tiles[,c(3,4,1,2,5,6)]
return(tiles)
}
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